A Study of CC-95266 in Participants With Relapsed and/or Refractory Multiple Myeloma

Inclusion Criteria: * Age ≥ 18 years * Participant has a diagnosis of multiple myeloma (MM) with relapsed and/or refractory disease. Participants must have confirmed progressive disease (as per IMWG criteria) on or within 12 months of completing treatment with the last anti-myeloma treatment regimen before study entry or have confirmed progressive disease within 6 months prior to screening and who are subsequently determined to be refractory or non-responsive to their most recent anti-myeloma treatment regimen, except for participants with cellular therapy (e.g., Chimeric antigen receptor (CAR) T-cell therapy) as their last treatment, who may enroll beyond 12 months. * Participants in Part A, and Part B Cohort A, and Part B Cohort B must have received at least 3 prior anti-myeloma treatment regimens (note: induction with or without hematopoietic stem cell transplant (HSCT) and with or without maintenance therapy is considered one regimen).Subjects in Part B Cohort C only must have received at least 1 but not greater than 3 prior anti-myeloma treatment regimens, including a proteasome inhibitor and immunomodulatory agent including: * Autologous HSCT, unless the subject was ineligible * A regimen that included an immunomodulatory agent (e.g., thalidomide, lenalidomide, pomalidomide) and a proteasome inhibitor (e.g., bortezomib, carfilzomib, ixazomib), either alone or combination * Anti-CD38 (e.g., daratumumab), either alone or combination. Subjects in Cohort C do not require prior anti-CD38 antibody therapy. * Measurable disease * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Adequate organ function Exclusion Criteria: * Known active or history of central nervous system (CNS) involvement of MM * Active or history of plasma cell leukemia, Waldenstrom's macroglobulinemia, POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes) syndrome, or clinically significant amyloidosis * Active autoimmune disease requiring immunosuppressive therapy * History or presence of clinically significant CNS pathology such as seizure disorder, aphasia, stroke, severe brain injuries, dementia, Parkinson's disease, cerebellar disease, or psychosis Other protocol-defined inclusion/exclusion criteria apply.